In centrifugal pumps, as well as in axial pumps, unbalanced forces act on the impeller in the axial direction. This axial force on the impeller is counterbalanced by one of several methods, such as by providing double suction, a balancing drum, or a balancing disc. In practice, however, some slight unbalance still exists. Then, to prevent unstable impeller conditions, a single or a double directional axial thrust bearing is included in the pump to support the axial forces produced.
It is important that the calculated axial thrust values for a pump be verified by actual measurements to assure that the bearing loads are within allowable limits. Although manufacturers typically perform thrust measurements at room temperature for this reason, the manufacturers are not equipped to measure the thrust at temperatures and pressures typical of those encountered with pumps circulating fluid in a nuclear reactor.
One type of axial thrust measuring device is used by the Lawrence Pump and Engine Company. This device is designed for use in an ambient atmosphere essentially at room temperature and nonpressurized (open tank suction). In order to use this device a short extension rod is welded to the impeller bolt. This extension rod thus rotates with the pump impeller and is supported by an outboard bearing at the end of an inlet Tee on the suction side. A force measurement of axial thrust is accomplished using a force transducer at the end of the rotating shaft and a manual lever arm is used to apply an axial load to the rotating shaft. A sufficient load is applied to the rotating extension to provide a lift-off force of the rotor from the associated axial thrust bearing.
The Lawrence Pump and Engine Company device was an in-house fixture designed for use at room temperature plus the differential pressure of the pump only, whereas the subject invention is designed for use at a differential pressure of 2000 p.s.i. or more, and at elevated temperatures, i.e., temperatures substantially higher than room temperatures. In addition, this the subject invention uses a fixed shaft as opposed to a rotating shaft in the "Lawrence device".
Disclosed in U.S. Pat. No. 4,287,758 (Swearingen) is an apparatus for measuring thrust loads on a rotatably mounted shaft in contact with a rotor of a centrifugal compressor. The shaft is subjected to the loads from the rotor, and a proximity transducer monitors the actual movement of the shaft to determine a thrust loading readout. The use of an LVDT (Linear Voltage Differential Transformer) to measure the end thrust or torque of a rotating shaft is disclosed in U.S. Pat. No. 4,182,168 (Desch). The use of load cells to measure axial force of a thrust bearing housing against an outer casing is disclosed in U.S. Pat. No. 3,763,700 (MacDonald). This axial force measurement is made while the rotor of a steam turbine is rotating.
In U.S. Pat. No. 3,665,758 (Tiller), a device for measuring the thrust preload applied by spring washers against bearings mounted on a rotatable armature shaft of a high speed motor is disclosed. The exposed end of the rotatable shaft is aligned with and coupled to a rod extending to a load cell. Other apparatuses of general interest which are used for measuring torque are disclosed in the following U.S. Pat. Nos. 3,782,186 (Burch et al); 3,828,610 (Swearingen); 4,432,246 (Granat); and 4,435,988 (Corry et al).